1. Field of the Invention
This invention relates to coating apparatus for applying a coating of an insulating lacquer to steel strip, especially electrical steel strip. The invention also relates to methods of applying coatings using this apparatus, and to steel strip coated thereby.
2. Description of Related Art
It is known to coat non-oriented electrical steel strip on one or both surfaces with an insulation lacquer in order to improve the performance of the strip when employed, for example, in a magnetic core of an electrical machine; insulation coatings on electrical steel laminations reduce the eddy currents in a stack and hence reduce power losses in the steel.
Certain applications for steel require coatings which are essentially organic in nature and exhibit only reasonably good punchability characteristics. Such coatings, however, exhibit good welding properties. Conversely, other coatings comprise inorganic additives and exhibit excellent welding characteristics but behave poorly when punched. Therefore the coating chosen has to be a compromise to suit the final application required.
U.S. Pat. No. 4,288,492 discloses an electrical steel sheet coated with an insulating lacquer of an organic polymeric material. The thickness of the lacquer coating is less than 10 microns, typically between 1 and 8 microns. U.S. Pat. No. 4,288,492 does not disclose the method by which the coating is applied, other than by reference to the employment of a roll coater. Conventional roll coaters generally comprise a grooved roller which rotates in contact with the surface of a sheet to be coated and applies a controlled pressure thereto. Coatings applied by such rolls tend to be uneven and the grooved rollers themselves are susceptible to damage and blockage because of pick-up resulting in subsequent poor coating of the electrical steel surface. U.S. Pat. No. 4,288,492 also discloses the step of curing the applied coating with ultraviolet rays or electron beams.
It is extremely difficult with conventional electric steel coatings to produce the very thin coatings of uniform thickness required because of the rheology of the applied coating.
In order to be suitable for some methods of application such as the multiple-step kneading roll method, coatings require the lubricity imparted by organic additives. However, high organic solvent based coatings have many disadvantages such as inflammability, the need for expensive after burners for burning off solvent vapour and the need to remove volatile organic compounds.
Other methods of coating such as the pick-up roll method are limited to low speed coating because it is impossible to pick up high viscosity liquids onto the pick-up roll. This method has further disadvantages, for example, the thickness of the resulting coating becomes irregular if there are any variations in the coating speed.
A system for applying coatings to substrates using a roller having an engraved surface is shown in "Raster- und Haschurenwalzen hergestellt mittels Lasertechnik" in COATING 12/95, St. Gallen CH, and means for heating the substrate and apparatus to lower the viscosity of the lacquer is shown in U.S. Pat. No. 4,949,667.